Generically the object of hydrometallurgical electrodeposition processes is the physical transfer of positively charged metallic ions from the electrolyte which contains them dissolved in a given concentration, to the submerged surfaces of negative charged energized cathodes. The basic electrolytic cells is composed of two energized electrodes—typically flat conducting plates, hanging parallel at a given distance in the electrolyte—an anode of positive charge and a cathode of negative charge—which generate respective chemical reactions—oxidizing at the anode and reducing at the cathode. Upon applying a low voltage, continuous current to the anode, the anions (ions of negative charge) present in the electrolyte migrate to the anode, while the cations (metallic ions positively charged) migrate the cathode where they deposit on the cathodic surface. The running of the process obeys Faraday's laws, whereby the chemical reaction is proportional to the flow of electrical charges on the plates of the electrodes—measured in amperes per unit of electrode surfaces—and referred to as current density. The current density is the key parameter that characterizes both the electrodeposition of metal in solution and its distribution on the cathode, as well as the efficiency of electrical current usage. The maximum electric efficiency is obtained operating the process at the maximum current density compatible with the continuity of metallic electrodeposition at the given sustained, acceptable level of quality. On the other hand, the current density is also limited in practice by the maximum diffusion of the metallic ions in said electrolyte at its given temperature. Actually, at a higher current density than that diffusion limit the stocks of metallic ions randomly distributed in the layers of electrolyte close to the cathode plates become exhausted, according to a concentration gradient decreasing towards the cathode plates, and therefore, the instantaneous availability for electrodeposition on the plate became insufficient to sustain indefinitely either the continuity of the process or the resulting quality of the metallic deposit.
To better understand the problems associated with hydrometallurgical electrodeposition processes as industrial scale, the electrolytic cells can be visualized as being composed of the sum of individual basic electrolytic cells—one after the other, disposed as productive units in series—physically filling the internal volume of each industrial electrolytic cell container. The electrochemical reactions and the physic-chemical phenomena of diffusion of metallic ions between each pair of plates anode/cathode facing each other in each basic cell is essentially similar, although not identical in magnitude in time, each basic cell in an industrial electrolytic cell behaves individually in accordance with it owns electrical, chemical, hydrodynamic given variables in its immediate surrounding, and for that reason, the result of metallic quality electrodeposition varies from cathode to cathode from each electrolytic cell at harvest. In order to improve the result at the level of an industrial cell it becomes essential to monitor and control the instantaneous variables of the process in each basic cell in real time.
For the continuous running of the industrial process in time, the concentration of metallic ions in the electrolyte within each basic cell must be maintained stable, within a given range. This condition is achieved by continuously feeding an appropriate flow of fresh electrolyte of high metallic concentration through one of the cell ends, allowing it to circulate in contact with the cathodic surface of the basic cells disposed in series, with the corresponding simultaneous discharge of the same flow of spent electrolyte or lower metallic concentration through the opposite wall or overflow side of the industrial cell.
While the electrochemical processes of electrowinning of non ferrous metals are run in the basic electrolytic cells, on the plate of the anode—manufactured typically with lead alloys which are insoluble in electrolyte good electrical conductors, structurally rigid and resistant to acid attack—some chemical substances are detached or generated, which are insoluble in electrolyte and of higher density than the electrolyte, and deposit on the bottoms of the cell containers as anodic sludge. The accumulation of anodic sludge requires empting the cell containers for periodic cleaning of the bottoms. In the case of copper, de-sludging prevents the hydrodynamic flow of the electrolyte close to the upper level of the sludge accumulated on the bottom from entraining the lighter sludge particles and mixing them in the trajectory of metallic ions flowing towards the cathode plates, introducing, in this manner, foreign particles into the pure metallic copper deposit required. In the case of the electrorefining processes, particularly copper, the cast impure copper anodes are soluble in the electrolyte, and contained impurities and traces of noble metals such as Au, Pt, Co and exotic metals such as Rhenium, etc, which by virtue of their extremely high value need to be recovered from anodic sludge upon its discharge from the containers, in subsequent extractions.
To obtain homogenous and uniform metal deposits in each cathode of each basic cell during production cycle of the processes of the electrowinning and electrorefining of non ferrous metals, it is essential to establish and maintain given current density as uniform as possible in the entire cathodic surfaces, and that condition requires maintaining simultaneously perfect parallelisms with the given uniform separation between all the point in the surfaces facing each other in the electrode plates, optimal electrical contact of each electrode with each electrical busbar and control of the temperature in each one of these contacts. To succeed in maintaining optimal electrical contact in time, it is indispensable to rely on the fact that the hanger bars of the electrodes and there respective plates will be in perfect geometrical condition, and maintain the electrical contact of the hanger bars with the busbar uninterrupted and free from interferences through permanent, frequent and thorough cleaning of the critical areas of these electrical contacts, with abundant washing with demineralized water.
At present, to reach the nominal capacity of metal in an industrial electrowinning or electrorefining plant of non ferrous metals, the electrolytic cell containers of the respective processes of hydrometallurgical electrodeposition are disposed in groups of cells forming banks or sections, each one composed of given number of containers, all uniformly dimensioned to install in their interior a given number of electrode, anodes and in particular cathodes, on whose surfaces the ions of metals will be deposited.
On the other hand, the design of the plant, the volume flow of the hydraulic electrolyte circuit and the power of the continuous current rectifier in the electrical system to energize the cells in their banks are dimensioned so as to obtain the nominal capacity of metal electrodeposition assuming sustained application during the entire operational cycle, of given current intensity per unit of cathodic surfaces installed in the containers of the cells. As electrodeposition is a process of continuous aggregation in time of metallic ions on the cathodic surface energized inside the cells, and thereby, the application of current from the time of immersion of the empty cathodes until the harvest of metal from the full cathodes—is maintained according to the real evolution in time of the variables of the specific process of the electrodeposition in each cells during the cycle—until reaching a convenient given average weight of metal accumulated in the cathodes. Essentially, the operational management of the process of electrodeposition in each basic cell has as an objective permanent and stable management of three fundamental parameters in electrodeposition, in such a way as to maintain them in optimum, sustained equilibrium from the beginning to the end of each operational cycle: the volume flow of electrolyte at the given temperature at the given concentration of metal in solution, the total available anodic and cathodic surface effectively energized in the cell, and the given current density uniformly applied to those energized cathodic surfaces.
In industry, at present none of these parameters and neither their instantaneous evolution in time is measured simultaneously in each cell and in real time.
To form the bank, the containers are installed adjacent to each other with their longitudinal lateral wall close together, in such a way that the respective longitudinal axis are parallel and positioned at right angles with respect to the longitudinal axis of the plant building. After connecting the respective hydraulic and electrical circuits with their equipment, the containers grouped in banks become banks of operational electrolytic cells in the plant. The banks are disposed forming two or more parallel lines along the longitudinal direction of the plant covering its surface.
Traveling cranes mounted transverse above the cell banks run in the longitudinal sense of the plant covering its surface for the transport, manipulation, insertion of the empty cathode blanks in any cell, and also for the removal, transport and manipulation of the harvested full cathodes from each cell at the beginning at the end, respectively, of each productive cycle. Industrially, the banks of cells are started and operated in such a manner that the harvests of cathodes from the respective cells are sequenced in time to maximize the use of the traveling cranes.
At present, in the electrolytic cells of industrial hydrometallurgical electrodeposition processes of electrowinning and refining of non ferrous metals, the electrodes are energized with continuous current of high amperage and low voltage, by means of direct mechanical contacts with the electrical busbars, which are typically of machined, high purity copper. The electrical busbars are disposed longitudinally parallel between each other directly supported on electrical insulators installed over the upper edges of the lateral walls of adjacent cells in their bank. The electrodes are laminar, flat plate electrical conductors which hang transverse to the cells by means of hanger bars that project outwards from the upper vertices of the plates, made of solid copper or steel shapes with a conducting facing or lining for efficient electrical contact with the busbar. The electrodes are installed transverse to the longitudinal axis of the cells, parallel and uniformly spaced from each other, anodes and cathodes intercalated, supported on spacer electrical insulators which maintain them equidistant. The length of the electrode hanger bar is supplied to suit the width of each cell so as to reach and contact the electrical busbars disposed at both sides of each cell.
To force the passage of continuous electrical current from the anode to the cathode hanging immersed in the electrolyte solution with ions of a non ferrous metal, the points of electrical contact between the ends of each electrode hanger bar with the electric current busbar on the lateral walls of the electrolytic cells are disposed alternated. In effect, one end of the hanger bar of the first anode is in contact with the first electrical busbar, while the other end of the hanger bar of the same anode must remain electrically insulated to positively not make contact with the second busbar. The second electrical busbar must make contact with the hanger bar of the next adjacent cathode, at the opposite end, immediately contiguous to the contact of the hanger bar of the first anode, and must remain electrically isolated from the first busbar. Schematically, in the electrical circuit of the electrolytic processes of interest, the electrical current enters the electrolyte from the electric busbar typically through end in contact with the hanger bar of the first anode, down through the plate of the submerged anode, then crossing electrically the ionized solution of electrolyte and making contact with the submerged plate of the next adjacent cathode, then returning from electrolyte to the second electrical busbar through the hanger bar of the cathode in contact with it. In the electrowinning processes of non ferrous metals where the anodes are insoluble, the unit electrical scheme for “n” anodes installed in each cell and their respective “n−1” cathodes intercalated in between the anodes, assure that both faces of the cathodic plate in each basic cell are supplied with metallic ions from the respective adjacent anodes. In the processes of electrorefining, where the anodes are made of impure metal and soluble in the electrolyte, the unit electrical scheme is repeated for “n” cathodes installed with the respective “n−1” anodes intercalated in between the cathodes.
Typically, for electrowinning of non ferrous metal, especially copper, solutions of the metal and sulfuric acid are utilized as electrolytes, in volumes flows that are related with their temperature, and principally, with the industrial current density imposed to the electrodes. In the case of copper, typically the volume flows are in the range of 14 to 30 m3/hr of electrolyte at 45-50° C. for current densities between 250 and 500 amperes per square meter, enabling to electrodeposit metallic copper at a rate between 6-10 gr/minute per square meter of cathodic surface.
During the production cycle in copper electrowinning, specially when the cells are operating with high flows, high electrolyte temperature and high current density to the electrodes, abundant oxygen is generated at the anode and some hydrogen at the cathode of each basic cell, gases which climb and emerge from the electrolyte surface into the plant atmosphere, carrying significant volumes of sulfuric acid as acid mist which is very toxic to human health. To comply with the admissible limits of contaminant substances in suspension in industrial plants indicated by the current environmental legislation, copper electrowinning cells of the latest design are operated covered and are equipped with hoods or equivalent collector devices for the collection, control and management of acid mist. The anti-mist devices are installed longitudinally supported on top of the electrode hanger bars, or alternatively, over the upper edges of the frontal walls of each cell, so that their inferior footprint perimeter remain above the electrodes. To harvest full cathodes at the end of the production cycle in each cell, the hood or equivalent anti mist capture device must be removed with the crane, and reinstalled after reloading the cell with empty cathode blanks before restarting the next production cycle.
In the electrorefining processes of non ferrous metal, especially copper, the impure metal to be refined is first melted and molded in laminar plates which are monolithic with their hanger, and said soluble plates positioned in the electrolyte as anodes in the electrolytic cell. The electrolyte also contains sulfuric acid and copper in solution, just as in the processes of electrowinning just described. In the copper electrorefining processes generally the volume flows of electrolyte at 62-65° C. vary between 14 to 18 m3/h (and current densities between 250 to 320 amperes per square meter), and are lower compared to the corresponding values in copper electrowinning. The lower flows and current densities generate much smaller volumes of acid mist than in electrowinning, whereby copper electrorefining plants generally are able to comply with environmental legislation through good ventilation without need of special collector hoods.
In the industrial operation of electrolytic cells, electrical short circuits are occasionally produced by direct contact of the laminar plates of the electrode, which are of particular relevance by the problem they impose by localized high temperatures, above 500° C., generated by high amperage currents in the electrical contacts of the hanger bars and electrical busbars. In effect, prior art electrical insulator polymer composite materials used in the areas of non contact supports of electrode hanger bars with electrical busbar are formulated with high contents of binding resin and with global contents inorganic reinforcements in general insufficient, and moreover of design, and shapes generally inappropriate. Starting from temperatures above 90-100° C., the thermal expansion of state of the art polymer composite material used in spacer insulators, specially structurally reinforced in the longitudinal sense with pultruded reinforcement bars (whose coefficient of lineal expansion is not compatible with the coefficient of lineal expansion of the polymer composite material of the electrical insulator which they reinforce) begin to bend and thereby start loosing their dimensional stability. This dimensional and geometrical instability of the insulator causes displacements in the positions of the electrodes, thereby favoring the continuity of short circuit initiated, prolonging them in time; and thereby, increasing the probability of generating additional short circuits upon carbonization of the binding resin of the insulators at the resulting high temperatures. Heat disintegrates the resin binder of the insulator material and thereby electrical insulation can collapse, resulting in fires or other accidents and irreversible damages. Notwithstanding the material deficiency commented, the use of pultruded bars in structural reinforcement of electrical insulator of polymer composite material for electrolytic cells continues widespread in the present art as can be reviewed in U.S. Pat. Nos. 4,213,842; 5,645,701; 7,204,919. It is indispensable for the industry to have available electrical insulators for electrolytic cells specifically constructed for better tolerance to occasional high temperature service, and of course, with sufficient thermal resistance to survive high amperage prolonged short circuits, and moreover, also internally structured for sufficient dimensional stability to maintain their geometry during such severe thermal episodes.
With the aforementioned in terms of absence of means to measure process variables and some basic equipment deficiencies, it becomes evident the truly overwhelming complexity of achieving equilibriums between electrical, thermal, physical, chemical, metallurgical, and hydrodynamic flow variables in the vicinity of immersed cathodes in each basic cell. The operational problem does not only consist in achieving satisfactory equilibriums with many changing variables but in the much bigger challenge of maintaining them substantially stable in time, from the beginning to the last instant of each production cycle, in each electrode of each industrial cell. In the present art, maintaining such equilibrium in the actual electrolytic cell is dictated by global empirical experience of the operators of each plant; said target equilibriums originally established and verified as suitable for the changing characteristics of Plant specific electrolytes. The correction or adjustment of variables is not as frequent a practice as is really required, and therefore, it is not surprising that the levels of electrodeposition performance and the usage of electrical energy observed in the industry at present remain quite below the possible theoretical optimum.
Perhaps the biggest technical problem at present is that in the basic electrolytic cells which conform the industrial cell, the instantaneous state of the variables of the electrolyte and the intensity and continuity of the electrical current to the process of the electrodeposition is not only not systematically measured, monitored, registered nor controlled in real time, but neither are instantaneous deviations or their trend in time diagnosed nor opportunely corrected with respect to their optimum. Such ability to measure, control and manage in real time is indispensable to optimize both the quality as well as the hydrometallurgical productivity of the electrodeposition processes in each basic cell, harvest after harvest, since not having opportunity to make adjustments in controlling the effectiveness, it is impossible to systematically assure before hand, the quantity and quality of the metal of the electrodeposit metal in the harvest cathode of the corresponding industrial cell at the end of each production cycle; and neither to improve consistently the global electrical performance with respect to present standards. The above problem can only be solved through technical management in real time, monitoring and managing simultaneously the unit behavior of each electrode in the basic electrolytic cell, in each industrial cell in the bank of cells and, certainly, also in the whole of industrial cells in the plant.
It is pertinent to point out that at present, for example, even for the experienced operator of electrowinning copper plants of the latest technologies as recently built in Chile in 2006, the lack of segregated information of the run cycle in real time, especially of the behavior of each anode and each cathode, per cell and per bank, prevents or at least hampers the controlled introduction of new hydrometallurgical technologies developed and in existence to increase electrolytic productivity and quality of the metal deposit. In fact, some operational technologies exist that are aimed to revert the primitive state of the present art of industrial plant operation management of processes of hydrometallurgical electrodeposition of non ferrous metal, such as Chilean Patent Application No 01057-2004 “Method for the evaluation and control of operational parameters of electrowinning or electrorefining of non ferrous metal plant” and the Patent Application No 02335-2003 “Support device to identify steel cathodes”, both assigned to 3M INNOVATIVE PROPERTY Co. USA. The contents and scopes of these patent applications although pointing in the correct direction, fall short, are partial and insufficient to supply effective means duly linked together to materialize segregated, measurements of variables by electrode in real time, at the basic cell level of electrolytic cells, industrial cells, banks of cells and of the whole of cells in a plant. Said condition appears as an essential base to opportunely detect any unfavorable deviations—at the very instant in which they start—and to correct them in such a way as to be able to maintain as normal the complex equilibriums of the variables of the processes of interest, at their optimum levels from the beginning to the end of each productive cycle in each and every cell.
Paradoxically, the electronic technology for measurement some parameters of the process in the basic electrolytic cell in real time also exist, for example, the vital measurement of the electrical current circulating in each cathode of the basic cell in a permanent manner in a real time, and the transmission of the data read from each for centralized computational management, which was conclusively and very successfully demonstrated at pilot industrial level in 2002. Moreover, the electronic circuit for instantaneous capture of the continuous current effectively circulating in the electrode of the basic cell in real time, its coding to electronic signals, its accumulation and transmission for computational management in a remote centralized system in the plant are claimed already in the Chilean Patent Application No 2789-2003. However the above mentioned technology to this date has not been applied industrially to the processes of interest in the industrial electrolytic cells in reference, fundamentally by lack of means that would allow bringing said electronic circuits sufficiently close to the electrodes in a stable manner, so as to insure ongoing correct operation. Friendly, non invasive, non disturbing means to the operational routines of the cells in the plant were lacking, as in fact occurred in 2002 pilot plant experience. To become industrially practical the means that are still lacking—and now are desired to patent—must be designed, adapted, concatenated and remain in convenient fixed positions in each basic cell, and at the same time, remain adequately protected to routinely operate in conjunction with industrial electrolytic cells.
With respect to electrical insulators, for properly energized, insulated and spaced electrodes in electrolytic cells, since Patent Application No 2385-1999 they have not been improved sufficiently. It has neither been incorporated in generalized fashion to the operational practices of industrial plants of hydrometallurgical electrodeposition, several other concepts and innovative technologies that improve metallurgical productivity and quality of metallic deposit with decreased usage of electrical energy. In fact, it has not been massively introduced, for example, concatenated means in the cell for decontamination of acid mist, increasing thermal performance, productivity and quality of the processes of electrowinning and electrorefining of non ferrous metals, taught in Patent Application No 527-2001, nor other more recent to increase productivity by improving the diffusion of metallic ions with controlled agitation of electrolyte as taught in Patent Application No 727-06. The delay in the introduction of innovative technology probably is due to attendant operational difficulties and certainly, in the present art that prevails in the hydrometallurgical copper industry of conservative operational caution, privileging what is prudent and demonstrated effective to produce stable volumes with assurance, over risking operational instabilities and uncertainty involved in the introduction of innovations in order to obtain promised benefits, which appear very difficult challenges to materialize not worth the risks.
The next step in the progress of the industry definitively points to the development of industrial operational protocols based on measurement of the variables and effective correction in real time of the problems of the processes of hydrometallurgical electrodeposition in the basic electrolytic cell—which is the real productive unit requiring control—as it should be and as is normal to expect in the 21st century of any massive industrial process of similar importance and complexity.